An ultrasonic welding method is disclosed in U.S. Pat. Nos. 8,052,816 and 9,486,955, in which the movement of the ultrasonic welding stack is delayed, following the initiation of the weld, until a predetermined condition is satisfied. During this delay, the speed of movement of the ultrasonic welding stack is zero as the stack is maintained at a stationary position. Similar methods for delaying motion, after some weld motion has already occurred, are disclosed in U.S. Pat. Nos. 9,144,937 and 9,849,628. Likewise, the speed of the ultrasonic welding stack in these methods is zero during the delays.
While the methods referenced above are beneficial in achieving high quality ultrasonic welds in the majority of applications, they can have undesirable consequences in some applications. In particular, in cases where the plastic material softens very quickly once ultrasonic energy is imparted to the workpieces being welded, the force between the workpieces decreases rapidly. This decrease can continue even after the predetermined condition for moving the ultrasonic welding stack is satisfied since this movement does not occur instantaneously due to various electrical and mechanical limitations of the motion system used to drive the movement of the stack, such as the limited rate at which electrical current can be ramped up in a motor. If the contact force between the workpieces becomes too low, the ultrasonic vibrations are not transmitted properly from the stack to the weld joint, potentially resulting in scuffing or marking of the surface of the workpiece in contact with the ultrasonic horn, a notable increase in audible noise during the welding process, and a reduction in the consistency of weld quality.